1. Field of the Invention
The present invention relates to a pump control apparatus for controlling operation of a motor-driven pump which is mounted on a vehicle and is PWM-controlled.
2. Description of Related Art
In a vehicle, various kinds of motor-driven pumps (fuel pump, water pump, oil pump, etc.) are mounted, operations of which are controlled by PWM control (Pulse Width Modulation Control). For instance, for achieving high-precise fuel pump control by a fuel pump control apparatus arranged to control operation of a fuel pump, a pump control signal has to be converted to a signal appropriate for actual operation of the pump. Specifically, when a low-frequency duty signal (a PWM control signal) calculated by an engine controller at every predetermined interrupt is to be output, the low-frequency duty signal is converted to a high-frequency signal by the fuel pump control apparatus. In this case, by pump driving operation based on the high-frequency signal, a pump motor is switched at high speed and at earlier intervals than the time constant (the response to a speed command) of the pump motor, thus providing a smooth operation of a fuel pump (see JP7(1995)-317620A). It is to be noted that the same control as for the fuel pump is executed on pumps such as a water pump other than the fuel pump.
In the aforementioned pump control apparatus, however, the frequency of a PWM control signal is changed to be high for achieving high-speed switching of the pump motor. This would cause a problem that the control apparatus (particularly, a drive circuit) generates high heat. For avoiding such problem, a special countermeasure for heat radiation is required.
Here, in order to avoid heat generation of the control apparatus (particularly, the drive circuit), the frequency of the PWM control signal has only to be set to be low. However, when the frequency of the PWM control signal is low, pump noise would increase (the frequency falls in an audible range).